1. Field of the Invention
The present invention relates to a magnetic recording head having a function of inducing magnetization reversal by applying a high frequency field to a magnetic recording medium.
2. Background Art
In recent years, recording density of a magnetic recording/reading device such as a hard disk drive (HDD) has been requested to increase as fast as 40% per year. The surface recording density is expected to reach 1 Tbits/inch2 by the year 2013.
To improve the surface recording density, it is required to miniaturize a magnetic recording head and reading head and reduce the particle size of a magnetic recording medium. However, the write field strength is reduced by miniaturization of the magnetic recording head, which may cause insufficient recording capability. Also, the influence of thermal fluctuation becomes considerable by reducing the particle size of the magnetic recording medium. Accordingly, it is required to increase the coercivity and the anisotropic energy when reducing the particle size of the magnetic recording medium, which is difficult in practice. Thus, improvement of the recording capability is the key to improvement of the surface recording density.
To address this problem, an assisted recording technique, in which heat or a high frequency field is applied to temporarily reduce a coercivity of a magnetic recording medium only when recording information, has been suggested.
An assisted recording technique of applying a high frequency field is referred to as “microwave assisted magnetic recording” (MAMR) and receives much attention recently. In MAMR, a high frequency field in high microwave bands is applied to a nanometer region to locally excite a recording medium and reduce a magnetization switching field for recording information. Since the magnetic resonance is used according to this technique, using a strong high frequency field having a frequency that is proportional to the anisotropy field of the recording medium is required to sufficiently reduce the magnetization switching field.
JP Patent Publication (Kokai) No. 2005-025831A discloses a high frequency oscillator that sandwiches a laminated film, which is structured similarly to a GMR device (giant magneto resistive device), between electrodes for generating a high frequency assisted magnetic field. The high frequency oscillator can generate a tiny high frequency oscillation magnetic field by injecting a conduction electron having spin fluctuation generated in a GMR structure into a magnetic material via a nonmagnetic material.
JP Patent Publication (Kokai) No. 2010-257539A and JP Patent Publication (Kokai) No. 2010-182361A disclose a structure in which an interval between a main pole and an oscillator at a position above an air bearing surface is larger than that on the air bearing surface. By reducing a magnetic field applied to the oscillator from the main pole, the oscillator easily oscillates and loss of write field strength is reduced.
“Microwave Assisted Magnetic Recording”, J-G. Zhu et. al, IEEE trans. Magn., Vol. 44, No. 1, pp. 125 (2008) discloses a technique of recording information on a magnetic recording medium having a large magnetic anisotropy by providing a field generation layer (FGL) that rapidly rotates as a result of spin-torque near a magnetic recording medium adjacent to a main pole of a vertical magnetic head and generating a microwave (high frequency field).
“Medium damping constant and performance characteristics in microwave assisted magnetic recording with circular as field”, Y. Wang et. al, Journal of Applied Physics, Vol. 105, pp. 07B902 (2009) discloses a technique of effectively assisting magnetization reversal of a magnetic recording medium by providing an oscillator between a main pole of a magnetic recording head and a shield on a rear side of the main pole (trailing side) and changing a rotational direction of a high frequency field depending on a polarity of a write field.